1. Field of the Invention This invention relates to article theft detection and more particularly it concerns novel apparatus for electronically detecting the passage of protected articles through an interrogation zone such as the exit from a store or other protected area.
2. Description of the Prior Art
U.S. Pat. No. 3,740,742 to Thomas F. Thompson and Joseph W. Griffith describes apparatus for detecting the passage of a resonant electronic responder circuit through an aisle in a store through which customers must pass. Plates or coils are provided along the aisle and then are energized with pulses to produce sharp electrostatic or electromagnetic pulses in the aisle. These pulses cause resonant electrical responder circuits, attached to the protected articles carried through the aisle, to resonate for a duration following each pulse. A receiver is provided to detect the resultant radiation from the resonant responder circuits and the receiver is gated to detect signals only after the energizing pulse has terminated.
Other apparatus which detect resonant electrical responder circuits by generating pulses and monitoring the resulting radiation from the resonating circuits are shown and described in U.S. Pat. Nos. 2,812,427, 2,899,546, 2,958,781, 3,117,277, 3,218,638, 3,299,424, 3,363,246, 3,363,247, 3,373,425, 3,440,633 and 3,740,742.
Similar resonant responder circuit detection techniques as applied to medical diagnosis are described in U.K. Pat. No. 906,006 and in Publications entitled "Medical Electronics: The Pill that `Talks`" by H. E. Haynes and A. L. Witchey, pp. 52-54, RCA Engineer, Vol. 5, No. 5, February-March 1960 and "Telemetering of Intraenteric Pressure in Man by an Externally Energized Wireless Capsule" by John T. Farrar, Carl Berkley and Vladimir K. Zworykin p. 1814, Science, Vol. 131, June 17, 1960.
In addition, U.S. Pat. No. 4,476,459 in the name of Michael N. Cooper describes a pulsed detection arrangement wherein the characteristic decay of the signal from the resonant responder circuit is monitored and utilized to distinguish the circuit from other energy sources which may produce the same frequency.
In all of the foregoing pulsed detection arrangements the pulses of electromagnetic energy are generated either by turning an oscillator on and off or by causing a sudden flow of current through a transmitter antenna coil or through a pair of electrostatic antenna plates. The devices which utilize oscillators usually incorporate a tuned transmitter antenna circuit having a Q value substantially less than that of the resonant responder circuits. These devices are complex and expensive. They also require relatively long pulsing intervals and therefore the amount of time available for monitoring the decaying resonance of the responder circuits is limited. The devices which cause a sudden flow of current through a coil or a pair of plates have the advantage of simplicity and economy. Also, these devices produce an interrogation pulse which lasts less than one cycle of the responder circuit resonant frequency and so provide maximum time to monitor the response. However, the frequency spectrum of the interrogation pulse is quite wide and a large amount of energy is wasted in generating unused frequency components.
Prior art responder detection arrangements also utilize either a common antenna for both generation of the pulsed electromagnetic field and for reception of resonant circuit responses or they use separate transmitter and receiver antennas. While the common antenna provides the advantage of being relatively simple and compact, the separate transmitter and receiver antennas are preferable because the transmitter antenna should be in the form of a simple loop coil to maximize pulse energy throughout the interrogation region while the receiver antenna should be in the form of dual cancelling coils to protect against interfering radiation from remote sources. Separate transmitter and receiver antennas are usually arranged on opposite sides of an aisleway although it has been proposed, for example in French Pat. No. 763,681 and U.S. Pat. Nos. 3,169,242 and 3,765,007 to locate them adjacent each other. However, such an arrangement requires a complex and cumbersome supporting structure. It has also been known to provide self supporting antennas in the form of metal pipes or bands, for example, as shown in U.S. Pat. Nos. 4,384,281, 3,820,103 and 3,820,104 and British Pat. No. 1,085,704; and it has also been proposed to mount an antenna inside a metal pipe, for example as shown in U.S. Pat. No. 4,251,808. None of these arrangements, however, permit the effective integral mounting of separate transmitter and antennas in a simple structure.